The present invention relates to switching amplifier and regulator architectures, and more specifically reducing DC offset voltages in such architectures.
In virtually any amplification system, a standard design goal is the minimization of DC offset voltage. DC offset voltage is defined as a non-zero DC voltage observed at the amplifier output when zero DC voltage is applied to the input. In an audio amplifier, the DC offset voltage can appear suddenly at the output terminals (and thus at the speakers) at the instant the amplifier is energized or activated, producing an unpleasant thump or pop. This phenomenon is observed in both linear and switching (class-D) amplifiers and comes from a variety of sources.
Minimization of DC offset voltage can be achieved by the use of carefully matched circuit elements, by adaptive (i.e., self-adjusting) mechanisms, or both. Some amplification systems are designed with relays between their output stages and the speakers that are open at the time the amplifier is energized and close only after a very slow acting continuous time servo loop has had sufficient time to null the output offset. The use of relays is costly, however, and can impact reliability as well. The slow acting servo loop also requires a time constant that is large (it must be significantly greater than the period of the lowest audio frequency being amplified) and is therefore difficult to integrate onto a silicon chip. Digital implementation of this same form of slow, real-time servo loop requires a large number of bits (e.g., 16 or more) which is also prohibitive.
Other digital solutions have proven to be highly effective in reducing or eliminating both transient and steady-state DC offset voltages in a variety of applications. Such solutions are described in U.S. Pat. No. 6,724,248 for DC OFFSET SELF-CALIBRATION SYSTEM FOR A DIGITAL SWITCHING AMPLIFIER issued Apr. 20, 2004, and U.S. patent application Ser. No. 10/807,903 for DC OFFSET SELF-CALIBRATION SYSTEM FOR A SWITCHING AMPLIFIER filed Mar. 23, 2004, the entire disclosures of both of which are incorporated herein by reference for all purposes. However, not all applications, e.g., audio applications, require the ongoing mitigation of steady-state DC offset provided by such complex solutions. Rather, such applications could benefit in reduced area and complexity (and therefore cost) from a simpler, more straightforward approach to the problem of DC offset.